I just read in this months boards mag that they found with other brands of similar crossover models that it made a big difference to the feel if they moved the straps to the forward or back hole positions (i realise inboard/outboard is makes a huge difference) but inboard - forward/back?

I don't use my 2008 k96 enough to test this theory but have them set inboard and back.
For general bump and jump at 77kg do you guys have an optimum...front or back holes?

Moving the straps forward will give you more maneuverability and more control (at the expense of top speed), but you really need to try for yourself to see/feel the difference.
You say you don't use your K96 enough. Don't try that when you are tired, out of shape and/or haven't had a session for a long time. Try moving them when you've had a few sessions, you know your sail and fin, and you know your board's behavior.

In addition to what Screamer said: For big sails of the boards sail range (6.5) outboard & double back straps is a big positive. For mid & smaller sails of the range, inboard or outboard work as Screamer describes. For the smallest of the sail range (4.5 or so) if on outboard straps, a small wave fin is problematic as the double rear straps encourages pushing off the fin which results in spin out. Single rear footstrap & small fin with a small sail is no problem.

cj0ne: If you're an experienced sailor, the front back footstrap trim for this type of board mostly comes down to personal preference. Generally, a wider spread can give advantages in wave riding since you can better drive the board through chop in a turn. But at the same time, a wider spread makes it a bit more difficult to stand relaxed on the board in a straightline. Apart from the spread we also have for aft tuning. Aft generally means more fin drive and also more sharp turning but at the expense of control.

If you feel like you don't have any particular problems getting into the straps or problems keeping the fin under control at speed and in chop or problems with spin outs in chop go for the rear back plug. If you have any of the problems above, try the forward plug. Then place the front straps where it feels the most comfy for you.